Demands for duplexers are increasing at a fast pace due to the rapid spread of wireless devices represented by mobile phones. Duplexers equipped with small acoustic wave elements having high steepness are strongly demanded, for example.
In recent years, a circuit configuration shown in FIG. 1 has been proposed as a ladder type filter (see Patent Documents 1 to 4, for example). In the configuration of FIG. 1, an inductance LP is connected in parallel to a series resonator S1 so that an attenuation pole can be generated in a region of a lower frequency than the filter passband.